The present disclosure generally relates to optical systems and methods for photometric measurement of samples and, in particular, to optical systems for photometric measurement of samples that comprise a light source comprising a plurality of light emitting elements and to a method of controlling the light source.
Various types of tests related to patient diagnosis and therapy can be performed by analysis of a patient's liquid sample. Such samples are typically placed in sample tubes. For analysis, they are extracted from the tubes, combined with various reagents, incubated, and analyzed. In typical clinical chemical and immunochemical analyses, one or more assay reagents are added to a liquid sample; the sample-reagent combination is mixed and incubated within an optical cuvette. In the course of the reaction, a change in optical properties occurs such as, for example, a variation of absorption, scattering or fluorescence of the sample. Normally an optical system is used to carry out a photometric measurement before, during and/or after the reaction, for example, by measuring the optical transmission through the cuvette using a beam of light generated by a light source and illuminating the sample-reagent combinations in such optical cuvettes. The results are used to generate extinction data, which are the ratio between light intensity input and output after passing the sample. In this way, the presence and/or concentration of analytes in a sample, which may be indicative of a diagnostic condition, can be determined by measuring response signals by a detector, typically at particular wavelengths. Examples of such photometric measurements comprise turbidimetric, fluorometric and absorption measurements and the like.
Different types of light sources may be used. These may comprise one or more light emitting elements. Examples of typically used light emitting elements are electric powered radiation sources such as incandescent lamps, electroluminescent lamps, gas discharge lamps, high-intensity discharge lamps, light emitting diodes (LEDs). Different types of light emitting elements but also light emitting elements of the same type, such as different LEDs, may emit light of different wavelengths. In particular, there are light emitting elements emitting light with a broad wavelength spectrum or with different respective wavelengths or wavelength bands.
It is known that temperature variations in a light source may cause changes in the emission intensity as well as shifts in the spectral emission. For example, the operational temperature of an LED, more specifically its junction temperature, has a direct influence on the wavelength of its light, the light intensity and the power resulting from the applied current. The junction temperature rises by heating due to dissipation of the electrical power, and is also affected by the ambient temperature. On the other hand, in order to perform a reliable and reproducible optical analysis it is important to maintain the emission intensity constant and to prevent spectral shifts.
For this reason, control of the temperature of the light source is normally necessary. As the junction temperature usually cannot be directly measured, the socket temperature can be used for temperature control of an LED, as it is closely related to the junction temperature.
Generally, the lifetime of a light source depends also on the temperature of the light emitting elements under operational conditions, i.e. when emitting light. Extending the lifetime of a light source is important because replacing the light source may be an expensive process, involving administrative overhead as well as labor. Moreover, it causes system downtime, during which time the system cannot be used. The total cost can thus be several times higher than the purchase cost of the light source itself. The life time of a light source comprising LEDs, for example, depends on the junction temperature under operational conditions. Lower junction temperatures and smaller temperature variations (less thermal stress) increase the lifetime of the whole LED. Moreover, different light emitting elements may have different lifetimes, i.e. a varying resistance to the operational conditions. This is also the case for different LEDs. In particular, LEDs emitting light of different wavelengths may have different lifetimes.
Controlling the temperature of a light source, especially cooling the light source, is therefore important also for extending the lifetime of light emitting elements.
Another possible way of extending the lifetime of a light source would be that of switching the light source off when not needed. However, switching the light source off and on again would cause temperature instabilities because of the time needed to reach a stable and reproducible operational temperature every time. Temperature instabilities cause in turn changes in emission intensities and spectral instabilities as described above. In order to prevent temperature instabilities, one possibility is to quickly pulse between on and off, for example, within microseconds. However, pulsing during a measurement leads to a loss of emission intensity, meaning less light available for the measurement during the measurement time, which is a drawback. Another possibility is to pre-heat the light source before switching the light source on again. This however requires temperature measurement and accurate temperature control, which may be complicated and difficult to obtain, especially in high-throughput systems.
Therefore, this is a need for an optical system and a method that can extend the lifetime of the light source while increasing temperature stability.